This invention is based on patent application Nos. 2000-133830 Pat. and 2000-133837 Pat. filed in Japan, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a liquid crystal display element (i.e., liquid crystal light modulation element) or more particularly to a liquid crystal display element which comprises a liquid crystal exhibiting a cholesteric phase at room temperature and which utilizes switching the liquid crystal to two stable states (bistable switching)
2. Description of the Background Art
In recent years, various researches have been made on liquid crystal display elements comprising a cholesteric liquid crystal such as a chiral nematic liquid crystal which is made to exhibit a cholesteric phase at room temperature by addition of a chiral material to a nematic liquid crystal.
Such liquid crystal display elements may be used, for example, as a liquid crystal display element of the reflection type utilizing the selective reflection of a cholesteric liquid crystal. The liquid crystal display elements of the reflection type may be used in monochrome image display utilizing a whitish color and a black or like background color to provide a substitute for paper or books.
In liquid crystal display elements utilizing the selective reflection of a chiral nematic liquid crystal, the peak of light reflectance of selective reflection in a wavelength-light reflection spectrum in a colored state is centered on a region around selective reflection wavelength. The spectral waveform of the portion is so sharp that when using a liquid crystal display element in a single layer, the obtained display is insufficient and unsatisfactory in that it is high in color purity (i.e., low in whiteness) and shows only a bright color.
In conventional liquid crystal display elements of the reflection type comprising a chiral nematic liquid crystal, for example, when monochrome image display is provided using a black or like background color, good monochrome image display has not been realized in the case of single-layer liquid crystal display elements, and the characteristics such as brightness, contrast and viewing angle dependency in image display are not fully satisfactory. Further, this type of liquid crystal display elements require good temperature compensation characteristics and extension of temperature compensation range.
Polymer stabilized liquid crystal display elements utilizing the selective reflection of a chiral nematic liquid crystal are proposed as a monochrome liquid crystal display element of the reflection type using a cholesteric liquid crystal (e.g., U.S. Pat. No. 5,847,798). In the proposed liquid crystal element, reportedly the reflection spectrum is made so broad in a state of selective reflection that monochrome image display is realized in a single-layer liquid crystal element.
However, in polymer stabilized liquid crystal display elements utilizing the selective reflection of a chiral nematic liquid crystal, a relatively high drive voltage is involved because the liquid crystal contains a network polymer (reticulated polymer), and the background color display characteristic (e.g. black color display characteristic) are insufficient and unsatisfactory in image display. Consequently, the contrast is low.
An object of the present invention is to provide a liquid crystal display element comprising a cholesteric liquid crystal such as a chiral nematic liquid crystal exhibiting a cholesteric phase, the display element being excellent in bistability, and capable of providing image display which is bright and high in visibility, so that in providing monochrome image display using a black or like background color, the characteristics such as sharp contrast, brightness and the like can be obtained.
Another object of the invention is to provide a liquid crystal display element comprising a cholesteric liquid crystal and enabling application of a low drive voltage.
A further object of the invention is to provide a liquid crystal display element comprising a chiral nematic liquid crystal and assuring a wide temperature compensation range.
The present inventors conducted extensive research to achieve the foregoing objects and found the following.
The liquid crystal display element proposed by the present inventors has a liquid crystal layer containing a cholesteric liquid crystal and held between a pair of substrates, the cholesteric liquid crystal being substantially free of a polymer. The proposed liquid crystal display element is capable of switching the liquid crystal to a colored state or to a scattering state by application of a specified voltage to provide image display. When the peak of selective reflection of the cholesteric liquid crystal is broad, in other words, when the peak of light reflectance of selective reflection in a wavelength-light reflection spectrum in a colored state is broad (extended to a wider visible wavelength range), good monochrome image display is realized using a whitish color and a black or like background color in a single-layer liquid crystal display element.
When generally a liquid crystal exhibiting a cholesteric phase is in a planar state wherein the helical axes of liquid crystal molecules are perpendicular to the substrate, the liquid crystal selectively reflects the light of a wavelength corresponding to a product of a helical pitch and an average refractive index of the liquid crystal. Accordingly, the liquid crystal would show, for example, a red, blue or green color if the liquid crystal has a selective reflection wavelength which corresponds to a red, blue or green wavelength range. By setting the liquid crystal, e.g. to an incompletely planar state, the peak of light reflectance of selective reflection can be made broad as peaks randomly occur due to a reflection direction of each domain (region).
According to the present inventors"" research, the following advantages are given to a liquid crystal display element having a liquid crystal layer containing a cholesteric liquid crystal between a pair of substrates, the cholesteric liquid crystal being substantially free of a polymer and being capable of switching to a colored state or to a scattering state by application of a specified voltage to provide image display.
When the peak of light reflectance of selective reflection in a wavelength-light reflectance spectrum in a colored state is broad and when an integral value in a visible wavelength range of a formula
{expxe2x88x92[2(xcexxe2x88x92nP)2/xcex94n2P2]}R
that is expressed as a function of wavelength xcex is in a range from about 40% to about 60% of an integral value in the visible wavelength range of a function f (xcex) that represents a spectral reflectance in the colored state, wherein n is an average refractive index of the cholesteric liquid crystal, P is a helical pitch of the cholesteric liquid crystal, R is a light reflectance in a selective reflection wavelength nP of the cholesteric liquid crystal in the colored state and xcex94n is a refractive index anisotropy of the cholesteric liquid crystal, image display is realized with a low color purity (i.e. high in whiteness) and is bright and excellent in visibility. For example, when monochrome image display is provided using a black or like background color, sharp contrast, brightness and like characteristics can be obtained. Further, the resulting display element is excellent in bistability and can be driven by application of a relatively low voltage.
The foregoing formula
{expxe2x88x92[2(xcexxe2x88x92nP)2/xcex94n2P2]}R
represents a normal distribution function wherein R is a light reflectance in a central wavelength (selective reflection wavelength nP) and (xcex94nP/2)2 is variance.
When an integral value in a visible wavelength range of the formula
{expxe2x88x92[2(xcexxe2x88x92nP)2/xcex94n2P2]}R
is more than 60% of an integral value in a visible wavelength region of a function f (xcex) representing a wavelength-light reflectance spectrum in a colored state, it is difficult to provide broad peak of light reflectance of selective reflection or in other words, the spectral waveform of peak portion of light reflectance becomes sharp, and image display tends to exhibit a high color purity and to show a brilliant color. On the other hand, if the integral value ratio is less than 40%, it is difficult to bring about the peak of light reflectance of selective reflection in a wavelength range of high luminous reflectance, e.g. a wavelength range in the range of about 500 nm to about 800 nm, resulting in a tendency of imparting low visibility. Consequently in any case of more than 60% or less than 40%, image display is low in visibility.
The present invention provides a first liquid crystal display element based on the above-mentioned novel findings.
(1) First Liquid Crystal Display Element
The present invention provides a first liquid crystal display element which comprises a pair of substrates at least one of which transmits light and a liquid crystal layer containing a cholesteric liquid crystal and held between the pair of substrates, the cholesteric liquid crystal being substantially free of a polymer (free of a polymer other than the liquid crystal), the display element being capable of switching the liquid crystal to a colored state or to a scattering state by application of a specified voltage (i.e. switching the liquid crystal to a colored state or to a scattering state in each pixel region) to provide image display, wherein an integral value in a visible wavelength range of the formula (expressed as a function of wavelength xcex)
{expxe2x88x92[2(xcexxe2x88x92nP)2/xcex94n2P2]}R
(wherein n is an average refractive index of the cholesteric liquid crystal, P is a helical pitch of the cholesteric liquid crystal, R is a light reflectance in the selective reflection wavelength nP of the cholesteric liquid crystal and xcex94n is a refractive index anisotropy of the cholesteric liquid crystal) is about 40% to about 60% of an integral value in a visible wavelength range of a function f (xcex) representing a wavelength-light reflectance spectrum in the colored state.
The inventor also found the following.
The following advantages are given to a liquid crystal display element which has a liquid crystal layer containing a chiral nematic liquid crystal and held between a pair of opposed substrates, the chiral nematic liquid crystal exhibiting a cholesteric phase. When the peak of light reflectance of selective reflection of the liquid crystal is broad (extended to a wider visible wavelength range), good monochrome image display is realized using a black or like background color in a single-layer liquid crystal display element.
As described above, when usually a liquid crystal exhibiting a cholesteric phase is in a planar state wherein the helical axes of liquid crystal molecules are perpendicular to a substrate, the liquid crystal selectively reflects the light of a wavelength corresponding to a product of a helical pitch and an average refractive index of the liquid crystal. Accordingly, the liquid crystal would show, for example, a red, blue or green color if the liquid crystal has a selective reflection wavelength corresponding to a red, blue or green wavelength range.
When the liquid crystal is interposed between a pair of substrates having a functional surface which is capable of adjusting an orientated state of liquid crystal molecules in the liquid crystal composition close to the substrate and an orientated state of liquid crystal molecules in the liquid crystal composition remote from the substrate to make the former orientated state different from the latter, the liquid crystal is set to an incompletely planar state, and thereby the peak of light reflectance of selective reflection can be rendered broad as peaks randomly occur due to a reflection direction of each domain (region).
According to the present inventors"" research, the following advantages are given to a liquid crystal display element which has a liquid crystal layer containing a liquid crystal composition and held between a pair of opposed substrates, wherein at least one of the substrates has a functional surface in contact with the liquid crystal composition which surface is capable of adjusting an orientated state of liquid crystal molecules in the liquid crystal composition close to the substrate and an orientated state of liquid crystal molecules in the liquid crystal composition remote from the substrate to make the former orientated state different from the latter. The liquid crystal composition is a chiral nematic liquid crystal exhibiting a cholesteric phase at room temperature. The chiral nematic liquid crystal comprises a nematic liquid crystal mixture of high refractive index (refractive index anisotropy 0.2 to 0.34) and at least one species of chiral materials added to the mixture (or at least one species of chiral materials added to the mixture for adjustment of selective reflection wavelength to a visible wavelength range). The liquid crystal composition has a phase transition temperature (temperature for transition from a cholesteric phase to an isotropic phase) Tch-I of 60xc2x0 C. or higher and shows a positive dielectric constant anisotropy so that the liquid crystal can be set to an incompletely planar state and the peak of light reflectance of selective reflection can be made broad. That is, the wavelength range of selectively reflected light covers the entire range in the range of 480 nm to 700 nm. Accordingly, image display is realized with low color purity (i.e. high in whiteness) and is bright and excellent in visibility. For example, when monochrome image display is provided using a background color such as black color, sharp contrast, brightness and like characteristics can be obtained. The resulting display element is excellent in bistability, and assures a wide temperature compensation range. The amount of the chiral material to be used is equal to an amount effective for forming a focal conic structure of the liquid crystal and a planar structure thereof.
The present invention provides the following second and third liquid crystal display elements based on the above-mentioned novel findings.
(2) Second Liquid Crystal Display Element
The second liquid crystal display element comprises a pair of opposed substrates and a liquid crystal layer containing a liquid crystal composition and held between the pair of substrates, at least one of the paired substrates having a functional surface in contact with the liquid crystal composition, the functional surface being capable of adjusting an orientated state of liquid crystal molecules in the liquid crystal composition close to the substrate and an orientated state of liquid crystal molecules in the liquid crystal composition remote from the substrate to make the former orientated state different from the latter. The liquid crystal composition is a chiral nematic liquid crystal exhibiting a cholesteric phase at room temperature and comprising a nematic liquid crystal mixture of refractive index anisotropy 0.2 to 0.34 and at least one species of chiral materials added to the mixture. The chiral nematic liquid crystal has a phase transition temperature of 60xc2x0 C. or higher and shows a positive dielectric constant anisotropy. The wavelength range of selectively reflected light in the liquid crystal layer covers the entire range in a range of 480 nm to 700 nm.
(3) Third Liquid Crystal Display Element
The third liquid crystal display element comprises a pair of substrates and a liquid crystal layer containing a liquid crystal composition and held between the pair of substrates, at least one of the paired substrates having a functional surface in contact with the liquid crystal composition, the functional surface being capable of adjusting an orientated state of liquid crystal molecules in the liquid crystal composition close to the substrate and an orientated state of liquid crystal molecules in the liquid crystal composition remote from the substrate to make the former orientated state different from the latter. The liquid crystal composition is a chiral nematic liquid crystal exhibiting a cholesteric phase at room temperature and comprising a nematic liquid crystal mixture of refractive index anisotropy 0.2 to 0.34 and at least one species of chiral materials added to the mixture for adjustment of selective reflection wavelength to a visible wavelength range. The chiral nematic liquid crystal has a phase transition temperature of 60xc2x0 C. or higher and shows a positive dielectric constant anisotropy. The wavelength range of selectively reflected light in the liquid crystal layer covers the entire range in a range of 480 nm to 700 nm.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.